This invention relates to ferroelectric memory devices and more particularly to screening processes for ferroelectric memories. The term ferroelectric is used to describe both a type of behavior and the group of substances exhibiting that behavior. Most solids undergo a rearrangement of electrical charge when a voltage is placed across the material. All the positive charges effectively shift slightly in one direction and the negative charges in the other. In ordinary materials, the charges will "relax" back to their original places when the voltage is removed. Ferroelectrics make up a special group of materials which can maintain the charge separation, or dipole, without an applied voltage. Furthermore, this dipole arrangement can be reversed by applying an electric field.
To better illustrate the properties of ferroelectrics, the unit cell of the ferroelectric barium titanate (BaTiO.sub.3) serves as an example. A unit cell is a geometric figure illustrating how the grouping of atoms which make up a solid are arranged relative to each other. The unit cell structure for a given solid is repeated throughout that solid.
The unit cell 4 for barium titanate shown in FIG. 1A has a cube structure. A barium (Ba.sup.2+) atom 6 is located at each of the eight corners of the cube. At the center of each cube face, is located an oxygen atom (O.sup.2-) 8. The radius of the six oxygen atoms project in toward the center of the cube leaving a octahedral space in the interior of the cube. In the approximate center of the unit cell, is located the titanium atom (Ti.sup.4+) 10.
When the barium titanate is heated above its Curie temperature of 120.degree. C., the unit cell is cubic and axes 12 shown by the dashed lines are of equal length. The Ti ion fits within the octahedral space and is centered within the cube as seen in FIG. 1A. However, when the barium titanate is cooled below 120.degree. C., the unit cell contracts. This Ti ion is now slightly larger than the octahedral space created by the oxygen ions. As a result, the Ti ion shifts to one side or the other of center within the interior space of the cube. The Ti ion can move in any of the six directions shown: vertically along axis 12z, horizontally along axis 12x, or in and out along axis 12y. In FIG. 1B, the Ti ion is located slightly below center and in FIG. 1C the Ti ion is located slightly above center.
The titanium ion has a plus four positive charge. Therefore, when the Ti ion is located slightly above center as in FIG. 1B, the top half of the cell is slightly positively charged when compared to the bottom half of the cell, a dipole is formed. The structure of FIG. 1B has a dipole of opposite polarity to that of FIG. 1A. Unit cells having dipoles aligned in a common direction are called domain. The BaTiO.sub.3 material of this example thus has six randomly possible domains.
FIG. 2 shows how these randomly oriented dipoles can be aligned in a common direction by applying an electrical force. Assume a crystal of barium titanate containing an equal number of positive and negative domains. Upon increasing the field (E) in the positive direction, the positive domains grow at the expense of the negative domains. The polarization (P) increases sharply (see OA) and reaches a saturation value (BC) when all of the domains are aligned in the direction of the field. The crystal now has a "single domain" structure. When the field E is again reduced to zero, a few domains remain aligned. At zero field in a definite value of polarization can be measured, and this is called the remanent polarization P.sub.r (OD). To reverse the remanent polarization, it is necessary to apply an electric field in the opposite (negative) direction. The field required for this purpose is called the coercive field, E.sub.c (OF). With further increase of field in the negative direction, uniform alignment of the dipoles again is reached, but this time in the direction opposite to the previous one (GH).
The polarization properties of ferroelectrics as represented by the hysteresis loop of FIG. 2 make ferroelectric films useful in the construction of certain nonvolatile memory devices. FIG. 3 shows such a ferroelectric memory device in cross section. The memory device shows a single memory cell in which a ferroelectric material 40 is located between a top electrode 42 and a bottom electrode 44, forming a capacitor. In the device of FIG. 3 the ferroelectric comprises lead zirconate titanate (PZT) but other ferroelectrics may be used. Ferroelectric 40 has been polarized to either a first (positive) polarity or second (negative) polarity depending upon the desired contents of the memory cell. When the cell had previously been polarized to a positive polarity, and the drive line 46 is pulsed high, a small displacement current flows and the bitline is pulled up to a small voltage and the cell reads a binary "zero". When the cell had been previously polarized to a negative polarity and the drive line 46 is pulsed high, a large displacement current flows and the bitline is pulled up to a higher voltage and the cell reads a binary "one". The difference in voltage is used to indicate a binary one or binary zero. Note that the polarity of the cell may be changed by the read process. Therefore, like a DRAM cell, the memory cell must be rewritten once it is read.
In the example immediately above, for the cell storing a binary one to be reliably read, the bitline voltage generated by the cell must be sufficiently greater than that expected from a cell storing a binary zero to distinguish the two states. The magnitude of the bitline voltage is a direct function on the magnitude of the remanent polarization.
To ensure the reliability of memory devices manufactured using ferroelectric materials, the devices are put through a preshipment screening procedure. Those devices having material and electrical defects are thereby discarded before shipping. Parts remaining after completion of screening procedures are then qualified for use for a given number of read cycles and a given maximum operating operating temperature.